Assesses the value of a recombinant human interferon beta product AVONEX in delaying the onset of clinically definite multiple sclerosis (CDMS) in subjects who have experienced a first and recent onset of a demyelinating event involving the optic nerve, spinal cord, or brainstem/cerebellum, and who are at risk for MS based on the presence of multiple magnetic resonance imaging (MRI) signal abnormalities in the brain.